Ternary System CsI-PbI2-BiI3 and Thermodynamic Stability of Cesium Metal Halide Perovskites

The thermochemistry of the ternary system CsI-PbI2-BiI3, of interest for applications in photovoltaics, memory devices, and nuclear applications, among other things, is investigated in this work. The binary phase diagrams CsI-PbI2 and CsI-BiI3 were subjected to renewed experimental investigation, and the compounds CsPbI3, Cs4PbI6, and Cs3Bi2I9 were found to be the only stable phases in the investigated temperature window. The liquidus lines and invariant equilibria were determined. The phase equilibria in the BiI3-PbI2 system were measured for the first time by using Differential Scanning Calorimetry (DSC). The end-members form a solid solution over the entire composition range. The pseudobinary section CsPbI3-Cs3Bi2I9 of the CsI-PbI2-BiI3 ternary system was moreover measured by DSC, as well as the ternary eutectic points. A thermodynamic model of the complete CsI-PbI2-BiI3 system was developed by using the Compound Energy Formalism (CEF) for the solid phases and the Modified Quasichemical Model in the Quadruplet Approximation (MQMQA) for the liquid phase. The binary systems were modeled first, and no ternary interaction parameters were found necessary to reproduce accurately the phase equilibria in the ternary system. With our model, the whole liquidus surface of the field CsI-PbI2-BiI3 is described for the first time.